1. Field of the Invention
The present invention relates to improvements in a spindle drive system of a machine tool.
2. Description of the Prior Art
FIG. 1 is a block diagram of a spindle drive system disclosed in the Japanese Patent Application No. 63-05164 SPINDLE DRIVE SYSTEM OF MACHINE TOOL, a prior filed application of the same applicant. In this drawing, numeral 1 denotes a numerical-control system (hereinafter referred to as the N/C system); numeral 2 is a spindle drive control unit (hereinafter referred to as the control unit); numeral 3 represents an induction motor (hereinafter referred to as the motor); numeral 4 expresses a speed sensor connected to the motor 3; numeral 5 denotes a spindle; numeral 6 denotes a transmission gearing which transmits the driving force of the motor 3 to the spindle 5; and numeral 7 denotes a position sensor which is a high-resolution sensor capable of detecting a rotational position of the spindle 5 to a positioning accuracy of 1/1000.degree. for example. The spindle drive system of the machine tool is constituted of the control unit 2, motor 3, speed sensor 4, and position sensor 7.
FIG. 2 is a block diagram showing the constitution of the control unit 2 comprising the major section of a conventional spindle drive system.
In FIG. 2, numeral 21 denotes a comparator which inputs a positional command .theta..gamma.* from the N/C system 1 and a position detection signal .theta..gamma., and outputs a position deviation signal .DELTA..theta..gamma. which is a difference between these signals. Numeral 22 is a position loop gain circuit which amplifies the positional deviation signal. Numeral 23 is a mode changeover switch as a control mode changeover means which makes the change and selection of a speed control mode for controlling the rotational speed of the spindle 5, that is, a turning operation mode for turning a rotating workpiece (not illustrated) to be machined, and a position control mode for controlling the rotational position of the spindle 5, that is, a C-axis operation mode. This switch is turned, in the speed control mode, to the "a" side for taking in a speed command .omega..gamma.* from the N/C system 1, and, in the position control mode, to the "b" side for forming a position control loop. Numeral 24 is a comparator which inputs an output signal of the mode changeover switch 23 and the speed detection signal .omega..gamma. from the speed sensor 4 and outputs a speed deviation signal .DELTA..omega..gamma. which is a difference between these signals inputted. Numeral 25 denotes a speed loop gain circuit which amplifies a speed deviation signal .DELTA..omega..gamma., and numeral 26 denotes an electric power conversion circuit which converts the output signal of the speed loop gain circuit 25 into power to be supplied to the motor 3.
In FIGS. 1 and 2, the speed command .omega..gamma.* outputted from the N/C system 1 is fed as a three-phase AC current command through the control unit 2 to the motor 3, which rotates in accordance with the speed command .omega..gamma.*. The rotation of the motor 3 is transmitted to the spindle 5 through the transmission gearing 6, the spindle 5 being driven. The gear ratio of the transmission gearing 6 is determined according to a purpose of operation. The position sensor 7 having a high resolution capable of detecting 1/1000.degree. is mounted on the spindle 5, thus detecting the position of the spindle 5 with a high accuracy and feeding back a detection result to the control unit 2.
The control unit 2, as described above, is so constituted as to control the speed of the spindle 5 in the turning operation mode and the position of the spindle 5 in the C-axis operation mode.
Next, the operation of the spindle drive system will be explained.
First, when operation is performed in the turning operation mode, that is, when ordinary turning operation using the spindle is carried out, the speed command .omega..gamma.* corresponding to a target speed of the spindle 5 is outputted from the N/C system 1, and the control unit 2 operates to make the speed .omega..gamma. of the motor 3 follow the speed command .omega..gamma.*. Namely, in the turning operation mode, the mode changeover switch 23 is set to the "a" side to take in the speed command .omega..gamma.* so that the speed loop control of the spindle 5 may be done by the control unit 2 shown in FIG. 2, and the comparator 24 receives the speed command .omega..gamma.* and the speed detection signal .omega..gamma. from the speed sensor 4, then outputting the speed deviation signal .DELTA..omega..gamma. which is a difference between them. This speed deviation signal .DELTA..omega..gamma. is amplified by the speed loop gain circuit 25, and converted into the power for driving the motor 3 by the power conversion circuit 26. The motor 3 is controlled to follow the speed command .omega..gamma.* from the N/C system 1.
Next, when the machine tool is to be operated in the C-axis operation mode, a command from the N/C system 1 is changed from the speed command .omega..gamma.* to the position command .theta..gamma.* of the C-axis operation mode. The control unit 2 drives the motor 3 in accordance with this position command .theta..gamma.*, thus controlling the position of the spindle 5. That is, in FIG. 2, the mode changeover switch 23 is set to the "b" side to form the position loop, while the comparator 21 receives the position command .theta..gamma.* from the N/C system 1 and the position detection signal .theta..gamma. from the position sensor 7, and outputs the position deviation signal .DELTA..theta..gamma.. This position deviation signal .DELTA..theta..gamma. is amplified by the position loop gain circuit 22, being inputted into the comparator 24 via the mode changeover switch 23. The comparator 24 and subsequent circuits operate in the similar manner as in the turning operation mode, being controlled so as to operate the spindle 5 in accordance with the position command .theta..gamma.* from the N/C system 1 through the motor 3.
In the above-described example, when the C-axis position control is performed by the use of the motor 3 with the turning operation mode changed to the C-axis operation mode, it is necessary to fully raise the positional loop gain and the speed loop gain in order to obtain responsivity enabling C-axis cutting. However, there has been such a problem that the mechanical system of the machine tool makes resonance depending on operating conditions during a rapid feed in the turning operation mode or in the C-axis operation mode, easily causing vibration and noise.